A relatively new process for making conventional steel, the bottom-blown oxygen steelmaking process, sometimes called the Q-BOP or OBM process, is beginning to receive considerable attention. Like the more conventional top-blown basic oxygen process (BOP process), the new bottom-blown oxygen process is a basic process utilizing a combination of an oxygen blow and lime-containing basic slag to remove impurities from the unrefined molten iron. Unlike the conventional top-blown process however, the new bottom-blown oxygen process blows oxygen through tuyeres extending through the vessel refractory lining below the molten metal surface. Each tuyere is substantially flush with the inside surface of the vessel refractory lining and is of a double-pipe construction wherein oxygen is blown through a central pipe which is surrounded by a larger concentric pipe for the simultaneous injection of a protective jacket fluid such as natural gas, propane or other gaseous or liquid fluid comprising or containing hydrocarbons. The hydrocarbon jacket fluid acts as a supercoolant, the hydrocarbon constituent endothermically dissociating to prevent a rapid increase in the metal temperature that would otherwise result from the oxidation reactions, and more importantly to cool the tuyeres and refractory material adjacent thereto to prevent the rapid erosion thereof.
Although the top-blown basic oxygen process utilizes only an oxygen blow, it has long been recognized that subsurface blowing with oxygen would not be commercially practicable. As noted above, subsurface oxidation reactions with pure oxygen are so violent and exothermic, that the molten iron would be heated to exceedingly high temperatures before the metal could be refined. Furthermore, any such subsurface tuyere for injecting only oxygen, and the refractories adjacent thereto would be very quickly burned away in a matter of seconds. As noted above, the bottom-blown oxygen process overcomes this problem by simultaneously injecting a jacket fluid which emerges from the tuyere concentrically surrounding the injected oxygen. Although practically any hydrocarbon would suffice as a jacket fluid, the most common jacket fluid used in the United States has been natural gas.
Experience with the bottom-blown oxygen process has shown that the ratio between the oxygen and the jacket fluid injection rates must be carefully controlled in order to control charge temperature and optimize tuyere and bottom refractory life. For example, using natural gas as the jacket fluid in conventional practices, it has been found that the natural gas injection rate should be within the range 5 to 10 volume percent of the oxygen injection rate, and preferably at approximately 8 volume percent. These ratios are for typical commercial practices wherein the original charge metal comprises 20 to 25 percent cold scrap. Since a considerable amount of heat is consumed during the first part of the blow in melting scrap, the scrap does act as a coolant, cooperating with the jacket fluid to maintain a reasonable charge temperature. The scrap does not however, have any appreciable cooling affect on the tuyeres and adjacent refractories. Here, the jacket fluid is the only effective coolant.
The new bottom-blown oxygen process has been widely acclaimed as having many advantages over prior art process, including its good appetite for scrap, i.e. the process typically consumes from 20-25 percent steel scrap. Although this feature was favorably regarded in the past when scrap was cheap and plentiful, more recent economic conditions have caused scrap prices to increase substantially, and scrap shortages have occurred in some locations. This has forced some steelmakers to look for practices which minimize the use of steel scrap. Although the amount of scrap that can be charged for making a heat in a bottom-blown oxygen vessel can be varied within limits, it has been found that scrap charges below about 20 percent of total charge metal do not lend themselves to efficient commercial operation. This is due to the fact, as noted above, that the scrap serves as a coolant in cooperation with the jacket fluid to keep the charge metal temperature at desired levels. As scrap contents are reduced below the 20 percent level, the final melt temperature after blowing is increased proportionally. Although it is possible to proportionally increase the injection rate of jacket fluid to counter-balance the reduced steel scrap charge, this tends to appreciably shorten the tuyere and vessel bottom life, because even modest increases in jacket fluid flow rates may cause localized over-cooling at the tuyere outlets causing molten charge metal to solidify at the outlet and eventually plug the tuyere.